Vitamin D is a prohormone for the daughter metabolite 1 alpha,,25(OH)2D3, which is a steroid hormone. As a consequence of being highly conformationally flexible, la,25(OH)2D3 presents a wide variety of shapes to its cellular environment. This in turn allows the generation of a variety of biological responses by activation of signal transduction pathways via one or more receptors involved in the production of either classic genotropic transcription of genes and/or rapid non-geriotropic biological responses. The 1 alpha,25(OH)2D3-mediated transcription of genes has been well studied and it is known to regulate a variety of cells and tissues. Conversely, the non-genotropic responses being activated by 1alpha,25(OH)2D3 appear to be mediated by a different signal pathway since the time frame of these responses which occur from seconds to minutes contrary to the genotropic frame of hours. Additionally, these types of responses can be separated from each other using a variety of path or action-specific derived analogs for 1 alpha,25(OH)2D3. The relevance to this proposal occurs in the study of the latter mechanism, in which a putative membrane receptor is believed to be responsible for mediating these responses. Thus, I propose to isolate and biochemical characterize the protein(s) responsible of 1 a,25(OH)2D3-mediated nongenotropic responses in plasma membrane invaginations or vesicles called caveolae. In other systems, caveolae have been linked to other steroid mediated rapid responses in which receptors to the respective ligands were shown to reside. We aim to isolate and purify the proposed membrane receptor which selectively binds 1 a,25(OH)2D3 with high affinity. This followed by characterization of the protein, leading to cloning and expression to learn its biological properties.